The 3:1 descent ratio can be used as a backup to which reference?

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Multiple Choice

The 3:1 descent ratio can be used as a backup to which reference?

A practical descent planning rule is being tested here: when you don’t have reliable precise descent guidance from the flight computer, you can estimate a descent path using a simple ratio. The 3:1 descent ratio means you descend 1,000 feet for every 3 nautical miles of horizontal travel. That gives a rough 3-degree glide-path feel and lets you map a feasible start of descent from your current altitude to the target altitude over the distance you have to go. This is best used as a backup to what the FMS provides in terms of top-of-descent points. The FMS TOD is the primary reference for where to begin the vertical profile, so you’d use the 3:1 rule only when TOD data aren’t available or trustworthy. It isn’t about manual altitude entries or fixed vertical-speed constraints; those are separate controls or constraints, whereas the 3:1 rule is a quick, distance-based way to estimate a descent path when the primary TOD reference isn’t usable. For example, to descend 6,000 ft over 18 NM, the 3:1 rule suggests starting descent to meet the target altitude around that distance (6 x 3 = 18).

A practical descent planning rule is being tested here: when you don’t have reliable precise descent guidance from the flight computer, you can estimate a descent path using a simple ratio. The 3:1 descent ratio means you descend 1,000 feet for every 3 nautical miles of horizontal travel. That gives a rough 3-degree glide-path feel and lets you map a feasible start of descent from your current altitude to the target altitude over the distance you have to go.

This is best used as a backup to what the FMS provides in terms of top-of-descent points. The FMS TOD is the primary reference for where to begin the vertical profile, so you’d use the 3:1 rule only when TOD data aren’t available or trustworthy. It isn’t about manual altitude entries or fixed vertical-speed constraints; those are separate controls or constraints, whereas the 3:1 rule is a quick, distance-based way to estimate a descent path when the primary TOD reference isn’t usable. For example, to descend 6,000 ft over 18 NM, the 3:1 rule suggests starting descent to meet the target altitude around that distance (6 x 3 = 18).

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